Chlorine-based bleach is the most widely used disinfectant in the world. The active ingredient of bleach, hypochlorous acid (HOCl) is also made by cells of the innate immune response, where it plays an important antimicrobial role in the mammalian host defense. However, when produced in excess in the host, HOCl is a devastating oxidant, responsible for tissue damage at sites of chronic inflammation and numerous disease states. Despite these various roles of HOCl as an antimicrobial agent and in disease, the physiological reasons behind the toxicity of HOCl are largely unknown. Moreover, the strategies that organisms have developed to defend against HOCl are unclear. Very recently, we discovered that bacteria encode redox regulated proteins, which use oxidation-sensitive cysteines to specifically sense and respond to HOCl with functional changes that enhance bacterial bleach survival. These results formed the basis for our working hypothesis; by understanding how bacteria sense HOCl and defend against HOCl-mediated damage, we will obtain an in-depth view of the physiological changes that occur in bleach-treated organisms, and uncover mechanisms that prevent and repair the damage that bleach inflicts. Through microarray analysis of bleach treated bacteria, we have identified two HOCl-specific transcriptional regulators in bacteria, the transcriptional repressor NemR, and the transcriptional activator YkgD. Both of these regulators contain highly conserved cysteines, which are likely involved in HOCl-sensing and responsible for the observed de-repression (NemR) or activation (YkgD) of target gene expression in response to bleach. We will now draw on our extensive expertise with redox-regulated proteins to investigate the redox-sensing mechanism of these novel transcriptional regulators. We will then characterize select downstream targets of NemR and YkgD that we have found to affect bacterial bleach resistance when deleted. These studies will significantly enhance our understanding about the cellular consequences of HOCl and may reveal potential means to manipulate HOCl sensitivity in both microbes and hosts. PUBLIC HEALTH RELEVANCE: Hypochlorous acid (HOCl), the active ingredient of household bleach, is a highly effective commercial disinfectant, and produced as key antimicrobial component by cells of the mammalian immune system to kill off invading bacteria. Overproduced at sites of chronic inflammation, however, HOCl leads to tissue damage and a variety of diseases. We will now elucidate the mechanisms that bacteria developed to sense and defend against HOCl-stress with the ultimate goal to devise strategies that increase bacterial bleach sensitivity and mitigate the damage that bleach causes in the host.